Various characteristics of a sintered ceramic material depends strongly on a fine texture thereof. Accordingly, ceramic sintering methods have been developed, selected and improved as various kinds of heat processing techniques so as to find a method of controlling the fine texture required for attaining aimed characteristics of a sintered ceramic material regarding various factors such as the kind of the raw ceramic powder, characteristics of the material powder, absence or presence of a sintering binder or material of ceramic in the pre-stage of sintering, for example, powder charging ratio of a molding product, crystal structure and physical properties of powder. One of the greatest subjects on the method of manufacturing such a sintered ceramic material is to establish a sintering method having controllability of a fine texture over a wide range capable of attaining aimed characteristics of the sintered ceramic material while ensuring high densification.
Means for dissolving the subject are generally classified into (i) development of a raw sintering powder having high quality and versatile nature, (ii) development of a powder processing technique including, for example, development of various kind of powder molding techniques while taking the performance and the productivity of the sintered material also into consideration, (iii) search for the fine texture controlling agent such as various kinds of binders and (iv) development for sintering and processing technique therefor including, for example, development of a heating method or heat treatment process, for which various research and development have been conducted. However, in an actual sintering process, since the existent powder processing technique and the heat sintering process technology are in an extremely complicate relationship the fine texture greatly varies locally or entirely or often lacks in the reproducibility, also in the sintering of a ceramic powder of constant powder characteristics and molding conditions, due to slight fluctuations of parameters in the sintering process (temperature, heating rate, etc.) or or slight changes such as of inevitable impurities in the powder and mold density.
On the other hand, for the manufacture of a sintered composite ceramic material, a method of sintering and densification by heating for a long period of time has been adopted usually. One of most prominent subjects in the conventional manufacturing method is that it is not possible to obtain a polyphase sintered ceramic material which is dense and comprises fine crystal grains and in which various kind of ceramic phase constitutions in the sintering material is sufficiently controlled in accordance with the purpose. For promoting the densification, there is a press-sintering method, for example, a hot press (HP) or hot isostatic press (HIP) method and, further a high pressure sintering method of applying heating under a higher pressure is also effective. On the other hand, for promoting the densification in a vacuum sintering method or an atmospheric pressure sintering method, various kinds of aids are used to attain the densification by a sort of polyphase composition. However, it is still difficult even by such a means to obtain a polyphase sintered ceramic material in which various kind of ceramic phase constitutions in the sintered material is sufficiently controlled and there has been the following problems.
(1) Crystal grains are grown coarser during the densification tending to leave pores. Further, the coarser growth of the crystal grains forms thermal stresses in the sintered material tending to cause destruction, particularly, in highly anisotropic ceramic.
(2) While addition of aids such as a grain size growth inhibitor is indispensable for preventing crystal grains from growing coarser during sintering under densification, the aids often reduce the characteristics of the sintered material. Further, even in a case of sufficiently modifying raw sintering material into a fine powder and preventing the crystal grain growth by low temperature sintering, it takes a long time for the densification and, also, control for the sintering temperature, time and atmosphere are extremely difficult.
(3) Although the crystal grains can be kept from growing coarser in the sintered material by the means (2) described above, the shape of each of the ceramic phases become indistinct or the attainment of the aimed composite phase constitution is remarkably restricted.
Lack of the controllability for the fine texture caused by the complicate relationship between the powder processing technique and the heat sintering processing technique involves the following problems.
(a) Conventional sintering methods represented, for example, by an atmospheric sintering or vacuum sintering requires a long time for heating and densification the material to be sintered and various kind of sintering mechanisms proceed stepwise or simultaneously in the course of the process. Accordingly, control for the fine texture is very difficult and the fine texture is determined unimeaningly depending on the conditions for attaining the densification of the sintering material (for example, temperature and time). As a result, the controllability for the fine texture is remarkably reduced.
(b) For the promotion and acceleration of the densification, press-sintering method under application of pressure can be adopted. However, in any of hot press (HP), hot isostatic press (HIP) and high pressure sintering method usually employed, since the heating method is similar to indirect heating like that in (a) above, provides relatively a slow heating rate and low controllability, application of the pressure is useful for removal of pores in the sintered ceramic material but does not function sufficiently for the control of the fine texture in the sintered material.
(c) For controlling the heating rate to the powder material to be sintered described in (a) and (b) above, it is considered to apply a current supply sintering method in which electric current is directly supplied to the powder to be sintered and the ohmic heat generation is utilized for heating. For instance, as a method of manufacturing a cermet or a conductive composite ceramic, a current supply hot press sintering method is utilized for shortening the time required for densification (Powder and Powder Metallurgy, vol. 32, No. 6, p 215-218). However, such a method is effective only for the sintering of conductive ceramic or a mixture of a conductive ceramic with a semiconductive or insulative ceramic of a particular composition and can not be applied to the sintering of semiconductive or insulative ceramic. In addition, there is also a drawback that the temperature in the ohmic heat generation varies depending on local fluctuations of the molding density of the sintered powder or the locallized variation of the electric resistance of the powder, thereby causing variation of the temperature distribution in the sintered material and it is difficult to obtain a homogeneous fine texture. In addition, since the electric resistance of the material to be sintered is usually low, the direct current supply heating for the conductive material to be sintered has many difficulties also from an industrial point of view, for example, installation of a great current supply system is indispensable.
(d) Various kinds of sintering binders or sintering aids have been developed as a controlling agent for the fine texture, but the effect of such additives on the sintering mechanism and anticipation for the effect obtained as the result on the characteristics of the sintered material are still insufficient and they are extremely groping. Accordingly, it is difficult to attain a development of new ceramic material having a controlled fine texture under the consideration of various kind of characteristics of the sintered material, unless various results of experiment are obtained and, in addition, there is a not attainable subject not yet overcome for the control of the fine texture in the heat-sintering process as shown in (a), (b) and (c) even when the above-mentioned binders are used.
(e) Although the powder processing technology has been progressed remarkably in recent years, it is extremely difficult, for example, to manufacture a ceramic powder of homogeneous grain size distribution and with no aggregation for the raw material powder in order to obtain a homogenous fine texture and the manufacturing cost is much expensive. In addition, although the technique for uniformly molding a raw material powder, particularly, fine raw material powder, having uniform powder characteristics has been developed partially, for example, as a hot molding technique, it has not yet been completed, and a great care is required for the control of the fine texture, that is, characteristics of the sintered material in the powder process and the heat-sintering process, and there is a difficulty that the characteristics of the sintering material varies greatly depending on slight fluctuations and variations even, if any, of process parameters.
As an attempt for dissolving the foregoing problems, there have been developed various kind of plasma sintering methods (Proceedings of the First International Symposium on Ceramic Components for Engine, 1983, p 710-715) and microwave sintering methods (Ceramic Bulletin, Vol. 68, No. 2, 1989, p 376-386) as another short time sintering method for the promotion of densification and control of the fine texture shown in the problems (a)-(c) described above. The plasma sintering method utilizes super high temperature possessed in plasmas for the sintering under the appropriate control of an atmosphere, which is a process capable of rapidly heating a material to be sintered at an extremely high energy efficiency and it has been reported that densification and the suppression for the grain growth during sintering were attained simultaneously mainly in oxide series ceramics. On the other hand, the microwave sintering method is a cold process using microwaves as a heating means, and it has a feature of generating heat at the inside of the material to be sintered. As a result, it has been reported that rapid and uniform heating was possible irrespective of the size of specimens and the homogenity of the fine texture of the sintered material can be improved.
As another short time sintering method combined with a pressure technology, a simultaneous synthesis and sintering method of ceramic referred to as High-Pressure, Self-Combustion Sintering for Ceramics has been developed by using a so-called SHS (Self-Propagating High Temperature Synthesis) which was studied since 1967 in USSR (refer to Japanese Patent Publication Sho 60-246270 and Comm. Am. Ceram. Soc., c-224-5, 1984, Nov.). The SHS method is a method of self-heating by using a burnable exothermic reaction mixture such as a thermit composition. For example, a ceramic material can be synthesized from a mixture of constituent elements for the ceramic material by using this method without external heating by using heat generating compound forming reaction between each of the elements. An attempt for the simultaneous synthesis and sintering method aims for eliminating pores in the ceramic material to be synthesized through the SHS method by the pressure and manufacturing a dense sintered material in several seconds and it has been reported that a TiB.sub.2 sintered material was manufactured under a pressure of 3 GPa only by electric ignition to a pressed mixture of Ti (titanium) and B (boron). The relative density and the hardness of the sintered material were 95% and 2000 kg/mm.sup.2 respectively.
Further, as another short time sintering method with application of pressure, a method of densifying and a compacting ceramic material by the combination of Explosive Shock Compaction method with the SHS method has been proposed (refer to U.S. Pat. No. 4,655,830 and Advanced Ceramic Material, Vol. 3, No. 3, p 288-90 (1988)). This method conducts simultaneous synthesis and sintering (type I) or explosure shock compaction and post-shock-heating (type II) of ceramic under a high impact pressure of about several tens GPa in a short period of time. For example, in a micro sec order and it has been reported that a sintered TiC material was synthesized starting from a powder mixture of Ti and carbon as raw material under application of an impact pressure greater than 45 GPa in the type I. The resultant sintered material thus is relatively porous and has micro-hardness of 500-700 kg/mm.sup.2. It has been also reported that TiC-Al.sub.2 O.sub.3 composite ceramic was synthesized by the application of an impact pressure of 45 GPa from a powder mixture of TiO.sub.2, carbon and aluminum as the raw material. The micro-hardness of the resultant sintered material was 500 to 700 kg/mm.sup.2, intergrain bonding was relatively weak and fine cracks were observed in some places. As the type II method, there has been reported, for example, that the compound exothermic reaction is added for the post-shock-heating element to the shock compaction method for SiC ceramic, in which shock-compaction and post-shock-heating were conducted in a structure comprising a sintered SiC ceramic molding product sandwiched with molding pellets of a mixture of Ti and C. The resultant sintered SiC material had a relative density of 99% and a micro-hardness of 28 GPa.
Further, a 2-step sintering method improved from the conventional sintering method has tended to attract an attention again in recent years for the improvement of single phase ceramic, in particular, the homogenity of the fine texture although this is a densification process requiring a long time reported by (L. C. De. Jonghe, et al). The feature of this method resides in applying a heat treatment at a low temperature (a temperature at which sintering does not proceed substantially) for homogenizing the fine texture preceding to the heat sintering step, and it has been reported that the homogenity of the fine texture was remarkably improved by this low temperature homogenizing treatment. However, a considerable portion of the mechanisms is still not apparent at present and a working example thereof is restricted to oxide ceramics like that in the plasma sintering method described above.
On the other hand, from a view point of the development for the powder processing technique, in the recent study related to the densification of the ceramic and the control of the fine texture, studies have been made vigorously on synthesis and sintering of super fine ceramic raw material powder by a gas phase technique such as plasma synthesis and synthesis and sintering of super fine ceramic raw material powder by a liquid phase method such as a sol-gel method. There has been reported that refinement of the covalent bonding super fine SiC ceramic particles manufactured by a R. F. Plasma CVD process in the sintering step could be promoted considerably with no addition of raw aids (B, C, Al, Be), by the refinement of the raw material powder and the development for the production of composite raw material powders and binder-containing raw material powder (Pre-Print for the Lecture of the Ceramic Society, Part, No. 1, p 427-428, 1986). It is considered that the phenomenon is caused as a result of the promoting effect for the diffusing reaction developed in the sintering step due to the increase of the powder ceramic activity and the increase of the specific surface area or the like by the refinement of the raw material powder. In the sintering of the SiC ceramic powder produced by plasma synthesis, it has been reported that the growth of the crystal grain size could be retained to about several micronmeters even if the hot press sintering temperature was elevated up to 2200.degree. C.
As another example of applying an active plasma-synthesized powder and a super fine powder produced by the gas/solid phase heterogeneous reaction regarding the promotion of densification, controllability for the fine texture and the improvement of the homogenity in less burnable ceramics, a result of the densification due to vacuum sintering and hot press sintering of TiB.sub.2 ceramic has been reported (Journal of the American Ceramic Society, Vol. 67, No. 3, p 207-212 and Advanced Ceramic Materials, described above, Vol. 1, No. 1, 1986, p 50-54). In both of them, the raw material powder is an agglomerate powder with a grain size of less than 1 .mu.m and an extremely active powder. In the vacuum sintering for the plasma-synthesized powder, increase of the relative density to 98-99% was attained by sintering at 1800.degree.-2300.degree. C. also with this less sinterable TiB.sub.2 ceramic. On the other hand, in the hot press sintering of a super fine TiB.sub.2 powder produced by a solid/gas heterogenous reaction, a sintered material with sufficiently controlled fine texture having a relative density of greater than 99% and a crystal grain size of 2 um was obtained under the presence of a slight amount of Fe and Ni (up to 0.4% in total).
Referring collectively to various kind of recent techniques tried for overcoming the foregoing problems, the technique for the sintering process is, for example, as described below.
(A) The current supply sintering method is effective for conductive ceramic or cermet but it is not applicable to insulative or semiconductive ceramic. Further, it is extremely difficult to simultaneously attain sufficient control for the crystal grain size and the ceramic constituent phase together with the densification.
(B) In the various kind of plasma sintering methods, for example, sintering of Al.sub.2 O.sub.3 ceramic using R.F. plasmas, there is a difficulty that the heat sintering temperature for the material to be sintered by or plasma heating greatly depends on the deposited amount of water, characteristics of the sintered material vary greatly and the like. Further, in this sintering method, the stability of an oscillator output, sintering time, as well as the stability of gas flow rate and gas pressure of Ar, N.sub.2 and H.sub.2 or the like give a remarkable effect on the density and the fine texture of the sintered material and it is difficult to simultaneously attain both the high density and the homogenous fine texture and the modification of the fine texture over a wide range.
(C) In the microwave sintering method, there is a principle bar regarding the selectivity for the sintering material that the sinterability is determined depending on the microwave interaction, that is, the degree of absorption by the powder to be sintered. That is, there is a drawback that a conductive ceramic material reflects microwaves making it impossible for heat sintering. Accordingly, for the nature of the material to be sintered, there is a difficulty in view of the design for the sintering material that selectivity is given only to a low loss insulator transparent to the microwave or a combination of a low loss insulator with a lossy insulator as an absorber.
(D) In the high-pressure self-combustion sintering method, since synthesis and press-sintering proceeds without external heating but only by the own compound exothermic reaction, extremely high temperature is formed during sintering synthesis to promote degasing from each of elements, by which the sintered material generally tends to become porous along with the reduction of the pressure. For reducing of the porosity, manufacture of cermet (ceramic phase+metal phase) sintered material has been tried in recent years (Summary of the Proceeding in Autumn Meeting, Society of Powder and Powder Metallurgy, 1986, p 42-43).
In addition, since the synthesis temperature is extremely high and the reaction rate is generally high owing to a so-called self combustion mode of conducting sintering synthesis mainly with the inter-element mixing of compounds forming extremely great heat of reaction, the fine texture is unimeaningly determined depending on the temperature of the synthesis reaction and the cooling rate determined by the heat formed by the inter-element reaction, making it difficult to control the fine texture over a wide range. Further, in the production of a composite ceramic phase, it is difficult to control the constituent phase depending on the purpose, even if the thermodynamic stability of the ceramic phase is taken into consideration.
(E) In the method of manufacturing a composite ceramic using the SHS method under an impact shock pressure, combustive sintering synthesis is possible owing to a sufficiently high pressure caused by impact shock waves and high temperature between particles even if the SHS reaction is not self-sustaining. However, since the time of applying the pressure is as short as micro seconds and generation of high temperature is mainly localized on the surface of the particles, it still involves a drawback that the SHS reaction is not completed or the sintered material is liable to be destroyed due to the occurrence of cracks during rapid (.mu. sec. order) pressure elimination. Further, due to the property of the self-combustion mode like that in the high-pressure self-combustion sintering method, and the short time pressure application in the order of micro seconds it is difficult to control the fine texture over a wide range even by the use of post-shock heating.
(F) For the 2-step sintering method, a considerable portion of the mechanism is not still apparent and, in many of reported examples, the anisotropy of the crystal structure of the material and the anisotropy for the grain growth during sintering are moderate. However, since this is a sintering method requiring a long time, even when a homogenzing heat treatment is applied at a low temperature thereby conducting densifying heat sintering, it has a difficulty that the homogenity of the fine texture is tended to be lost depending on the heating rate and the sintering time from the temperature for the homogenizing heat treatment to the main sintering temperature. In particular, in the sintering of less sinterable ceramic having a strong anisotropy, since the range for selecting the optimum heating parameters in the main sintering step is extremely narrow, it is almost impossible at present to attain the homogenity of the fine texture, versatile control and densification.
(G) In the homogenizing sintering method aiming for the densification and the control for the fine texture of the ceramic using a plasma-synthesized powder, powder synthesized by solid/gas phase heterogenous reaction and ceramic powder by the sol-gel method resulting from the development of the powder processing technique, since the powder becomes active due to super fine powderization, a great amount of inevitable impurities are introduced in non-oxide series ceramics and the moldability is reduced remarkably both for the oxide series and non-oxide series ceramics, use of the molding aid or the like is essential, tending to cause scattering in the micro molding density. These drawbacks give a significant effect on the promotion of the densification and the control for the fine texture in the sintering step to leave various problems such as bubbles are left or causing abnormal grain growth is brought about due to the inevitable impurities in the low temperature sintering that utilizes the powder activity.